Heat pipe

ABSTRACT

A heat pipe able to switch between operation and suspension of heat transport when used for a bottom heat type and able to prevent the condensed working medium from dropping to the evaporator side and a waste heat recovery system using the same are provided. A heat pipe having an evaporator set at one end of a tubular closed container and using heat of an outside high temperature part to cause the inside working medium to evaporate and a condenser set at the other end side of the closed container and radiating heat to an outside low temperature part to cause the evaporated working medium to condense, wherein the evaporator is arranged below the condenser and has a holding means holding the liquefied working medium condensed by the condenser along with an increase in the amount of heat received by the evaporator to prevent return to the evaporator.

TECHNICAL FIELD

The present invention relates to a heat pipe suitable for recovering the waste heat of exhaust gas of for example a vehicular internal combustion engine and using it for heating cooling water of the internal combustion engine and a waste heat recovery system using the same.

BACKGROUND ART

In the past, as a vehicular heating system using a heat pipe, for example, the one shown in Japanese Utility Model Publication (A) No. 59-16211 is known. This arranges an evaporator of the heat pipe (loop type) in an exhaust passage of the engine, arranges a condenser of the heat pipe at an air outlet side of a heating use heater core, uses the heat pipe to directly transport exhaust heat to the heating use air, and thereby tries to secure heating performance in a short time after engine startup.

In this vehicular heating system, the heat pipe is provided at its middle part with a valve means. When the heating use air temperature obtained by the temperature sensor becomes a predetermined value, the valve means is closed and heat transport by the heat pipe is stopped.

However, in the art described in the above Japanese Utility Model Publication (A) No. 59-16211, when arranging the evaporator of the heat pipe at the bottom side and the condenser at the top side, that is, when used as the so-called “bottom heat” type, if the drops of water condensed by the condenser at the time the heat transport operation stops drop to the evaporator side due to vibration etc. during vehicle operation, rapid evaporation occurs, so the internal pressure of the heat pipe rapidly rises and the heat pipe is liable to rupture or the repeated pressure fluctuation is liable to cause the heat pipe to break. This phenomenon is particularly remarkable in a wickless bottom heat type heat pipe.

DISCLOSURE OF THE INVENTION

An object of the present invention, in consideration of the above problem, is to provide a heat pipe able to switch between operation and suspension of heat transport when used for a bottom heat type and able to prevent the condensed working medium from dropping to the evaporator side and a waste heat recovery system using the same.

The present invention achieves the above object by employing the following technical means.

In a first aspect of the present invention, there is provided a heat pipe having an evaporator (110A) set at one end of a tubular closed container (111) and using heat of an outside high temperature part (11) to cause the inside working medium to evaporate and a condenser (110B) set at the other end of the closed container (111) and radiating heat to an outside low temperature part (30) to cause the evaporated working medium to condense, wherein the evaporator (110A) is arranged below the condenser (110B) and has a holding means (112) holding the liquefied working medium condensed by the condenser (110B) along with an increase in the amount of heat received by the evaporator (110A) to prevent return to the evaporator (110A).

Due to this, since return of the liquefied working medium along with an increase in the amount of heat received by the evaporator (110A) is prevented by the holding means (112), it becomes possible to switch between operation and suspension of heat transport by the simple configuration of the holding means (112) in a heat pipe (110) when used as a bottom heat type.

Here, the liquefied working medium at the time of suspension of heat transport is held by the holding means (112), so the liquefied working medium can be prevented from dropping down into the evaporator (110A) due to outside vibration etc. That is, since rapid evaporation at the evaporator (110A) side can be prevented, there is no longer the risk of the inside pressure of the heat pipe (110) rapidly rising and the heat pipe (110) rupturing or repeated pressure fluctuation causing the heat pipe (110) to break.

In a second aspect of the present invention, there is provided a heat pipe where the evaporator (110A) is arranged under the condenser (110B) and has a holding means (112) which holds the liquefied working medium condensed by the condenser (110B) when the temperature of the outside low temperature part (30) becomes a predetermined value or more so as to prevent return to the evaporator (110A), whereby an effect similar to the first aspect can be obtained.

The holding means (112), in a third aspect of the present invention, may be made a valve element (112) provided between the evaporator (110A) and the condenser (110B) and opening and closing a return passage (111 a) of the liquefied working medium.

In a fourth aspect of the present invention, the valve element (112) has a passage (112 b) through which a steam working medium evaporated by the evaporator (110A) circulates and a dam part (112 a) preventing outflow of the liquefied working medium to the passage (112 b) side.

Due to this, even after the valve element (112) is closed, the steam working medium from the evaporator (110A) passes through the passage (112 b) and reaches the condenser (110B) where it is condensed, so a rise in the inside pressure in the evaporator (110A) can be prevented. Further, the dam part (112 a) of the valve element (112) can reliably hold the liquefied working medium and prevent inflow from the passage (112 b) to the evaporator (110A).

In a fifth aspect of the present invention, the height of the dam part (112 a) is set higher by a predetermined amount than the level of the liquefied working medium prevented from flowing out to the passage (112 b) side.

Due to this, the valve element (112) of the fourth aspect may be made a specific aspect enabling the liquefied working medium to be prevented from dropping down into the evaporator (110A) due to outside vibration etc.

In a sixth aspect of the present invention, the valve element (112) is driven by heat expansion and heat shrinkage of the thermal wax (113) provided at the condenser (110B).

Due to this, the valve element (112) can be opened and closed in accordance with the temperature of the low temperature part (30) corresponding to the condenser (110B). That is, when desiring to keep the temperature of the low temperature part (30) down to a predetermined temperature, the valve element (112) can be closed at a predetermined temperature and heat transport of the heat pipe (110) can be stopped.

Further, in a seventh aspect of the present invention, the holding means may be made an area enlarging part enlarging the area of the surface of the inside wall of the condenser (110B).

Specifically, in an eighth aspect of the present invention, the area enlarging part preferably uses a wick.

A ninth aspect of the present invention relates to a waste heat recovery system using a heat pipe (110) of the first aspect, wherein an evaporator (110A) of a heat pipe (110) is arranged in an exhaust pipe (11) for circulation of exhaust gas of an internal combustion engine (10), a condenser (110B) is arranged in a cooling water passage (30) for circulation of cooling water of the internal combustion engine (10), and the heat pipe (110) is used for transporting waste heat of the exhaust gas to the cooling water.

Further, the reference numerals in parentheses of the above means show the correspondence with the specific means described in the embodiments later.

Below, the present invention will be better understood from the attached drawings and the description of the preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the state of the waste heat recovery system mounted in a vehicle.

FIG. 2 is a side view of a waste heat recovery system in a first embodiment.

FIG. 3 is a cross-sectional view in a part A-A of FIG. 2.

FIG. 4 is a cross-sectional view showing a valve element in a heat pipe in a first embodiment (valve opening state).

FIG. 5 is a cross-sectional view showing a valve element in a heat pipe in a first embodiment (valve closing state).

FIG. 6 is a perspective view of a valve element seen from the B direction in FIG. 4.

FIG. 7 is a graph showing an operating state and a suspended state of a heat transport function of a heat pipe with respect to the cooling water temperature.

FIG. 8 is a cross-sectional view showing a valve element in a heat pipe in a second embodiment (valve opening state).

FIG. 9 is a cross-sectional view showing a valve element in a heat pipe in a second embodiment (valve closing state).

FIG. 10 is a graph showing an amount of heat conduction to engine cooling water according to a waste heat recovery system in a second embodiment.

BEST MODE FOR WORKING THE INVENTION

(First Embodiment)

A waste heat recovery system 100 in a first embodiment of the present invention is applied to a vehicle (automobile) having an engine 10 as a drive source for operation. First, a specific configuration is explained below using FIG. 1 to FIG. 6. Further, FIG. 1 is a schematic view showing the state of the waste heat recovery system 100 mounted in a vehicle, FIG. 2 is a side view showing a waste heat recovery system 100, FIG. 3 is a cross-sectional view in a part A-A of FIG. 2, FIG. 4 and FIG. 5 are cross-sectional views showing a valve element 112 in a heat pipe 110, and FIG. 6 is a perspective view showing a valve element 112 seen from the B direction in FIG. 4.

As shown in FIG. 1, the engine 10 is a water cooled type internal combustion engine which has an exhaust pipe (corresponding to the high temperature part in the present invention) 11 from which exhaust gas after fuel is burned is exhausted. The exhaust pipe 11 is provided with a catalytic converter 12 for purifying the exhaust gas.

Further, the engine 10 has a radiator circuit 20 in which engine cooling water (hereinafter, cooling water) is circulated to cool the engine 10 and a heater circuit 30 using cooling water (warm water) as a heat source to heat the air-conditioning air.

The radiator circuit 20 is provided with a radiator 21. The radiator 21 cools the cooling water circulated by the water pump 22 by heat exchange with the outside air. Further, the radiator circuit 20 is provided with a bypass passage (not shown) through which the cooling water is circulated bypassing the radiator 21. A thermostat (not shown) is used to adjust the amount of the cooling water circulating through the radiator 21 and the amount of cooling water circulating through the bypass passage. In particular, at the time of engine warmup, the amount of cooling water at the bypass passage side is increased and engine warmup is promoted. That is, overcooling of the cooling water by the radiator 21 is prevented.

The heater circuit (corresponding to the low temperature part in the present invention and cooling water passage) 30 is provided with a heater core 31 as a heating use heat exchanger. The water pump 22 is used for circulation of the cooling water (warm water). The heater core 31 is arranged in an air-conditioning case of a not shown air-conditioning unit. Air-conditioning air blown by a blower is heated by heat exchange with the warm water.

The waste heat recovery system 100, as shown in FIG. 2 and FIG. 3, is comprised of a plurality of (here, three) heat pipes 110 at the outsides of which fins 120 are provided. One end of each heat pipe 110 (evaporator 110A) is arranged in an exhaust pipe part 130, while the other end (condenser 110B) is arranged in the water tank 140. The members forming the waste heat recovery system 100 (explained below) are comprised of a stainless steel material provided with a high corrosion resistance. The members are assembled, then are soldered together by solder material provided at the abutting parts and engaging parts.

Each heat pipe 110 has a container 111, valve element 112, and thermostat 113. Inside of the container 111, a working medium is sealed. The container (corresponding to the closed container in the present invention) 111 is comprised of a straight round tube which is used in a posture with its longitudinal direction oriented in the vertical direction. The top end side of the container 111 is opened. This opening is closed by fastening a thermostat 113 there. The thermostat 113 is a temperature sensing part in which thermal wax expanding and shrinking in accordance with the temperature is sealed. Further, the valve element 112 is the characterizing part in the present embodiment. Details will be explained later.

Further, each heat pipe 110 is provided with a not shown inlet. The inside of the heat pipe 110 is evacuated to a vacuum (reduced in pressure) from this inlet, then a working medium is sealed in them, then the inlet is sealed. The working medium used here is water. Water has a boiling point of usually (at one atmosphere) 100° C., but since the tube 110 is reduced in pressure (for example, to 0.04 atmosphere), the boiling point becomes 30 to 40° C. Further, the working medium used may also be, in addition to water, alcohol, a fluorocarbon, freon, etc.

Each heat pipe 110 of this configuration forms an evaporator 110A at the bottom side, a condenser 110B at the top side, and an insulating part 110C between the two 110A, 110B and functions as a bottom heat type.

A plurality of the heat pipes 110 are arranged. The outside walls of the parts of the heat pipes 110 corresponding to the evaporator 110A and condenser 110B have plate type fins 120 formed from a thin sheet material joined to them. Further, the evaporators 110A formed by the heat pipes 110 are arranged in the exhaust pipe part 130 forming a square cross-section duct, while the condensers 110B are arranged in a water tank 140 forming a box shaped vessel. Further, the water tank 140 has an inlet pipe 141 and outlet pipe 142 connected to the inside of the water tank 140 joined to it at facing sides.

Further, the characterizing part of this embodiment is the provision of a valve seat 111 b inside each heat pipe 110 and a valve element 112 connected from the thermostat 113 and able to sit on the valve seat 111 b.

As shown in FIG. 4 to FIG. 6, the valve seat 111 b is positioned between the evaporator 110A and condenser 110B of the heat pipe 110 (container 111) (insulating part 110C), sticks out from the inside wall surface 111 a of the container 111 to the axial center, and thereby forms a ring shape in the circumferential direction at the inside wall surface 111 a.

The valve element 112 (corresponding to the holding part in the present invention) is based on a disk shaped member. At the outer circumference, it has a vertical wall part (corresponding to the dam part of the present invention) 112 a forming a ring in cross-sectional shape extending in the vertical direction in the circumferential. Further, at the axial center side from the vertical wall part 112 a, a plurality of hole parts (corresponding to the passage in the present invention) 112 b are formed passing through the disk shaped part and forming fan shapes. Further, the disk shaped part is integrally provided at its axial center with a shaft 112 c which is connected to a thermostat 113 at the top side. The shaft 112 c abuts against the thermal wax in the thermostat 113 and is biased to the thermal wax side by a not shown spring member. Therefore, conditional on the temperature of the outside of the thermostat 113 (corresponding to the temperature of the low temperature part of the present invention, specifically, the temperature of the cooling water circulating through the water tank 140) not satisfying the predetermined temperature (corresponding to the predetermined value in the present invention, for example, 90° C.), shrinkage of the thermal wax causes the shaft 112 c to be biased by the spring member to the thermostat 113 side and the bottom end of the vertical wall part 112 a of the valve element 112 and the valve seat 111 b to separate (valve opening state) (FIG. 4).

Further, conditional on the temperature of the outside of the thermostat 113 being the predetermined temperature or more, the shaft 112 c is pushed by expansion of the thermal wax (expansion force of thermal wax overcoming biasing force of spring member) to the opposite thermostat side and the bottom end of the vertical wall part 112 a of the valve element 112 sits on the valve seat 111 b (valve closing state) (FIG. 5).

When the valve element 112 sits on the valve seat 111 b, the inside wall surface 111 a, valve seat 111 b, and vertical wall part 112 a form a space M opening upward. This space M, as explained later, forms a space in which condensed water condensed by the condenser 110B is held. The volume of the space M is set to be at least the maximum amount of the condensed water. In other words, the top end position of the vertical wall part 112 a is set to become higher by a predetermined amount than the level of the maximum amount of the condensed water.

The waste heat recovery system 100 is formed in the above way. The exhaust pipe part 130 is interposed in the exhaust pipe 11 forming the part becoming the downstream side of the catalytic converter 12. Further, the two pipes 141 and 142 of the water tank 140 are connected to the heater circuit 30 (FIG. 1).

Next, the operation based on the above configuration and the mode of action and effects of the same will be explained with addition of FIG. 7. FIG. 7 is a graph showing the operating state and suspended state of the heat transport function of a heat pipe 110 with respect to the cooling water temperature.

Along with the startup of the engine 10, the water pump 22 is activated. The cooling water circulates through the radiator circuit 20 and the heater circuit 30. The exhaust gas of the fuel burned by the engine 10 passes through the catalytic converter 12, flows through the exhaust pipe 11 and exhaust pipe part 130, passes the outside of the evaporator 110A of each heat pipe 110 in the waste heat recovery system 100, and is discharged to the atmosphere. Further, the cooling water circulating through the heater circuit 30 circulates through the inside of the water tank 140 and passes outside of the condenser 110B of each heat pipe 110.

In the waste heat recovery system 100, after the engine 10 is started and until the cooling water temperature reaches a predetermined temperature, as shown in FIG. 4, the water (working medium) in each heat pipe 110 receives heat from the exhaust gas flowing through the exhaust pipe part 130 and boils and vaporizes at the evaporator 110A to become steam. This rises through the inside of the heat pipe 110, passes through the hole parts 112 b of the valve element 112 and between the vertical wall part 112 a and the inside wall surface 111 a to flow into the condenser 110B. The steam flowing into the condenser 110B is cooled by the cooling water flowing through the inside of the water tank 140, becomes condensed water at the inside wall surface 111 a, descends due to gravity, and returns along the inside wall surface 111 ato the evaporator 110A. The inside wall surface 111 a forms a return passage by which the condensed water flows down and is returned.

In this way, the heat of the exhaust gas is transmitted to the water and transported from the evaporator 110A to the condenser 110B. When the steam is condensed by the condenser 110B, it is discharged as latent heat of condensation, whereby the cooling water flowing through the heater circuit 30 is heated (waste heat recovery operation). Further, part of the heat of the exhaust gas is transferred from the evaporator 110A to the condenser 110B due to heat conduction through the outside wall of the heat pipe 110.

Therefore, when the outside air temperature is relatively low or the cooling water temperature reaches a predetermined temperature after the engine 10 is started etc., waste heat recovery is executed by the heat pipe 110 (left side of time axis in FIG. 7), the cooling water is positively heated, and warmup of the engine 10 is promoted, so the friction loss of the engine 10 is reduced, the increase in fuel for improving the low temperature starting ability is suppressed, etc. and the fuel economy performance is improved. Further, the heating performance of the heater core 31 having the cooling water as a heat source is improved.

On the other hand, when the heat transport of the heat pipe 110 (or operating conditions of the engine 10 etc.) results in the cooling water temperature reaching a predetermined temperature or more, as shown in FIG. 5, the thermostat 113 causes the shaft 112 c to be pushed to the opposite thermostat side (white arrow in FIG. 5) and the vertical wall part 112 a of the valve element 112 to sit on the valve seat 111 b. This being the case, the condensed water condensed at the condenser 110B is held at the space M forming the outer circumference of the valve element 112 and return of the condensed water to the evaporator 110A is obstructed. After this, when evaporation at the evaporator 110A proceeds, the steam passes through the hole parts 112 b of the valve element 112 and flows into the condenser 110B, then is condensed at the condenser 110B. Further, when evaporation at the evaporator 110A proceeds, the water in the evaporator 110A completely evaporates and becomes steam, whereupon heat transport is stopped. That is, waste heat recovery is suspended (right side of time axis in FIG. 7, and heating of the cooling water is suspended (waste heat recovery suspended).

Therefore, if waste heat recovery is continued along with the elapse of time after startup of the engine 10 and while the cooling water temperature is rising, the cooling water temperature overly rises, the heat radiating ability of the radiator 20 is exceeded, and overheating results. By switching to suspension of waste heat recovery, this inconvenience is prevented.

Here, in the present embodiment, by providing each heat pipe 110 with a valve element 112 closed by the thermostat 113 and forming a space M at the outer circumference of the valve element 112 at the time of valve closing, the condensed water from the condenser 110B is held and return to the evaporator 110A is inhibited. In this way, in each heat pipe 110 used as a bottom heat type, operation and suspension of heat transport can be switched by this simple configuration.

Further, the condensed water at the time of suspension of heat transport is held at the space M by the valve element 112, but since the position of the top end of the vertical wall part 112 a is set sufficiently higher than the level of the held condensed water (higher by predetermined amount), it is possible to prevent condensed water from dropping down to the evaporator 110A due to vibration etc. of the vehicle. That is, rapid evaporation at the evaporator 110A side can be prevented, so there is no longer any risk of the inside pressure of the heat pipe 110 rapidly rising and the heat pipe 110 rupturing or of repeated pressure fluctuations causing the heat pipe 110 to break.

Further, since the valve element 112 is provided with hole parts 112 b, even after the valve element 112 is closed, steam from the evaporator 110A passes through the hole parts 112 b, reaches the condenser 110B, and is condensed, so a rise in the inside pressure of the evaporator 110A can be prevented.

Further, since the valve element 112 is opened and closed by the thermostat 113, heat transport of the heat pipe 110 can be suspended based on the cooling water temperature.

(Second Embodiment)

A second embodiment of the present invention is shown in FIG. 8 to FIG. 10. The second embodiment is comprised of the first embodiment simplified in the configuration of the valve element 112A. That is, in the art described in Patent Document 1 in the section on the Background Art, a temperature sensor or valve means is used for operating and suspending heat transport by the heat pipe. Further, in actuality, control means for opening and closing the valve means in accordance with the temperature from the temperature sensor also becomes necessary and therefore the cost becomes higher. This solves the problem and prevents condensed water from dropping down at the time of suspension of heat transport.

The container 111A of the heat pipe 110 is a sealed vessel closed from the top end side as well. The thermostat 113 is eliminated. Further, the valve element 112A is comprised of a disk shaped part formed with a vertical wall part 112 a and hole parts 112 b. The shaft 112 c is eliminated. That is, the valve element 112A is arranged in a free state above the valve seat 111 b without support from other members.

Further, the amount of water sealed in the heat pipe 110 is adjusted in advance to an amount completely evaporating in the evaporator 110A when the exhaust gas temperature (amount of waste head), which rises in accordance with the load of the engine 10, exceeds a predetermined exhaust gas temperature.

In the present embodiment, after the engine 10 is started, when the exhaust gas temperature accompanying the engine load becomes a predetermined exhaust gas temperature or less, the steam evaporated at the evaporator 110A rises, passes through the hole parts 112 b of the valve element 112A, and reaches the condenser 110B. At this time, due to the upward flow of steam (steam flow rate), the valve element 112A is lifted up further than the valve seat 111 b (valve opening state of FIG. 8).

The steam flowing into the condenser 110B is cooled by the cooling water flowing through the inside the water tank 140, becomes condensed water at the inside wall surface 111 a, descends due to gravity, and is returned along the inside wall surface 111 ato the evaporator 110A. Therefore, due to the heat transport function of the heat pipe 110, the cooling water is positively heated (left side of abscissa in FIG. 10, waste heat recovery operation).

On the other hand, when the exhaust gas temperature accompanying engine load exceeds a predetermined exhaust gas temperature (along with an increase in the amount of heat received by the evaporator 110A), the water of the evaporator 110A completely evaporates and there is no longer any upward flow of steam, so the valve element 112A sits on the valve seat 111 b (valve closing state of FIG. 8). Therefore, the condensed water condensed at the condenser 110B, like in the first embodiment, is held in the space at the outer circumference of the valve element 112A, the return of the condensed water to the evaporator 110A is inhibited, and heat transport is stopped (right side of abscissa in FIG. 10, waste heat recovery suspended).

After this, when the engine 10 stops being operated, the exhaust gas is no longer supplied to the evaporator 110A, so the evaporator 110A rapidly falls in temperature. Further, since the cooling water has a large heat capacity, it holds the high temperature (approximately 80° C.) for a while, this cooling water side (condenser 110B) becomes the evaporator of the heat pipe 110, the exhaust gas side (evaporator 110A) becomes the condenser, and the original operation is reversed. Further, the condensed water held in the space M gradually evaporates, passes through the hole parts 112 b of the valve element 112A, and condenses at the exhaust gas side (evaporator 110A), whereby the heat pipe 110 returns to the original state.

In the present embodiment, even if the cooling water temperature does not sufficiently rise, once the engine 10 engages in a high load operation and the exhaust gas temperature becomes higher than a predetermined exhaust gas temperature, there is the problem that the valve element 112A ends up sitting on the valve seat 111 b and return to the original state becomes difficult until the engine 10 is turned off once, but compared with the first embodiment, there is the merit of being far cheaper.

Further, in the present embodiment, the vertical wall part 112 a is provided at the outer circumference of the valve element 112A, but even if provided around the hole parts 112 b, the water holding function can be similarly secured.

(Other Embodiments)

In the second embodiment, the valve element 112A was used as the holding means of the condensed water, but instead of this it is also possible to use an area enlarging part enlarging the area of the inside wall surface of the condenser 110B. Specifically, the area enlarging part may be made a wick comprised of metal mesh, metal felt, foam metal, sintered metal, etc. An effect similar to the second embodiment can be obtained.

Further, the heat pipe 110 (container 111) was shaped as a round tube, but the invention is not limited to this and may also be made an angular tube, flat tube, multi-hole tube, etc.

Further, the explanation was given of a high temperature part comprised of the exhaust pipe 11, a low temperature part comprised of the heater circuit 30, and the heat of the exhaust gas being transported to the cooling water, but it is also possible to use the waste heat of the heat generating equipment for heating a predetermined location.

Note that the present invention was explained in detail based on specific embodiments, but a person skilled in the art can make various changes, modifications, etc. without departing from the claims and concept of the present invention. 

1. A heat pipe comprising an evaporator provided at one end of a tubular closed container, and using heat of an outside high temperature part to cause the inside working medium to evaporate and a condenser provided at the other end side of the closed container and radiating heat to an outside low temperature part to cause the evaporated working medium to condense, wherein the evaporator is arranged below the condenser and has a holding means holding the liquefied working medium condensed by the condenser along with an increase in the amount of heat received by the evaporator to prevent return to the evaporator.
 2. A heat pipe comprising an evaporator provided at one end of a tubular closed container, and using heat of an outside high temperature part to cause the inside working medium to evaporate and a condenser provided at the other end side of the closed container and radiating heat to an outside low temperature part to cause the evaporated working medium to condense, wherein the evaporator is arranged below the condenser and has a holding means holding the liquefied working medium condensed by the condenser when a temperature of said low temperature part becomes a predetermined value or more to prevent return to the evaporator.
 3. A heat pipe as set forth in claim 1, wherein said holding means is made a valve element provided between the evaporator and the condenser, and opening and closing a return passage of the liquefied working medium.
 4. A heat pipe as set forth in claim 3, wherein said valve element has a passage through which a steam working medium evaporated by the evaporator circulates and a dam part preventing outflow of the liquefied working medium to the passage side.
 5. A heat pipe as set forth in claim 4, wherein a height of the dam part is set higher by a predetermined amount than the level of the liquefied working medium prevented from flowing out to the passage side.
 6. A heat pipe as set forth in claim 3, wherein the valve element is driven by heat expansion and heat shrinkage of thermal wax provided at the condenser.
 7. A heat pipe as set forth in claim 1, wherein the holding means is made an area enlarging part enlarging the area of the surface of the inside wall of the condenser.
 8. A heat pipe as set forth in claim 7, wherein the area enlarging part is a wick.
 9. A waste heat recovery system using a heat pipe, wherein an evaporator of a heat pipe as set forth in claim 1 is arranged in an exhaust pipe for circulation of exhaust gas of an internal combustion engine, a condenser is arranged in a cooling water passage for circulation of cooling water of the internal combustion engine, and the heat pipe is used for transporting waste heat of the exhaust gas to the cooling water. 